Cathode for lithium air battery comprising hollow structure and method of manufacturing same

ABSTRACT

The present disclosure relates to a cathode for a lithium air battery and a method of manufacturing the same, and more particularly to a method of manufacturing a cathode for a lithium air battery, in which a hollow structure including a carbon material having a nitrogen functional group is synthesized through electrospinning of a thermally decomposable polymer, coating with a nitrogen-containing polymer and heat treatment, and is utilized without a binder as a cathode carbon material for a lithium air battery, thereby increasing the performance and lifespan of a lithium air battery.

CROSS REFERENCE TO RELATED APPLICATION

The present application claims priority based on Korean PatentApplication No. 10-2019-0060899, filed on May 23, 2019 in the KoreanIntellectual Property Office, the entire content of which isincorporated herein for all purposes by this reference.

BACKGROUND OF THE DISCLOSURE 1. Technical Field

The present disclosure relates to a cathode for a lithium air batterycomposed exclusively of a hollow structure without a binder, and to amethod of manufacturing the same.

2. Description of the Related Art

Due to the limitations of lithium ion batteries for use with recenttechnologies such as electric vehicles (EVs), hybrid electric vehicles(HEVs), etc., a next-generation lithium battery capable of solvingproblems with lithium ion batteries, such as the low energy density andlimited capacity thereof, is receiving attention these days.

As the next-generation lithium battery, a lithium air battery is asystem using oxygen in the air as a cathode active material. The lithiumair battery is expected to have better capacity and energy density thanthe lithium ion battery because it is able to receive an unlimitedsupply of oxygen from the air.

The performance of the lithium air battery is greatly dependent on theproperties of the materials used for the cathode. The lithium airbattery is charged and discharged through oxidation and reductionbetween lithium at the anode and oxygen at the cathode. At the time ofdischarge, lithium ions oxidized at the anode are transferred to thecathode through a separator membrane via the electrolyte, and meet thereduced oxygen ions at the cathode, thus producing lithium peroxide(Li₂O₂). The lithium peroxide does not dissolve in the electrolyte butaccumulates at the cathode due to the electrical non-conductivitythereof. Ultimately, if the lithium peroxide covers the surface of theelectrode and clogs the pores, the lithium air battery becomesnonfunctional because it is impossible to transfer electrons andsubstances.

Consequently, the performance and lifespan of the lithium air batteryare determined by the amount of lithium peroxide that is generated andstored in the cathode and the extent of decomposition thereof.Therefore, it is necessary to design a cathode material that is able toprevent the pores in the cathode from clogging with lithium peroxidegenerated and stored in the cathode and enables quick decomposition oflithium peroxide.

SUMMARY OF THE DISCLOSURE

Accordingly, an objective of the present disclosure is to provide alithium air battery having high capacity and a long lifespan.

The objectives of the present disclosure are not limited to theforegoing, and will be able to be clearly understood through thefollowing description and to be realized by the means described in theclaims and combinations thereof.

The present disclosure provides a cathode for a lithium air battery,comprising a sheet having a plurality of hollow structures that arerandomly entangled. One or more of, or all of, the plurality of hollowstructures can have a length of 0.5 μm to 300 μm and includes a carbonmaterial having a nitrogen functional group.

The nitrogen functional group may include graphic nitrogen(graphitic-N).

In some embodiments, the cathode may include no binder.

In other embodiments, the hollow structure may have a length of 1 μm to100 μm.

In still further embodiments, the hollow structure may have a diameterof 2 μm to 10 μm.

In addition, the present disclosure provides a method of manufacturing acathode for a lithium air battery, comprising preparing a fibrouspolymer by electrospinning a spinning solution including a thermallydecomposable polymer, manufacturing a fibrous structure comprising acore including the fibrous polymer and a sheath including anitrogen-containing polymer by coating the surface of the fibrouspolymer with the nitrogen-containing polymer, manufacturing a filmconfigured such that the fibrous structures are randomly entangled, andheat-treating the film.

The thermally decomposable polymer may be selected from the groupconsisting of polystyrene (PS), poly(methyl methacrylate) (PMMA) andcombinations thereof.

The nitrogen-containing polymer may be selected from the groupconsisting of polydopamine (PDA), polyacrylonitrile (PAN), polypyrrole(PPy), polyaniline (PANI) and combinations thereof.

The fibrous structure may be manufactured by adding the fibrous polymerto a polymer solution including at least one selected from the groupconsisting of a nitrogen-containing monomer, a nitrogen-containingpolymer and combinations thereof and performing stirring.

The fibrous structure may be manufactured by applying thenitrogen-containing polymer at a thickness of 10 nm to 100 nm on thesurface of the fibrous polymer.

The fibrous structure may be manufactured by performing coating of thesurface of the fibrous polymer with the nitrogen-containing polymer 1 to10 times.

The method may further comprise subjecting the fibrous structure toultrasonication before manufacturing the film.

The sheet, configured such that hollow structures including a carbonmaterial having a nitrogen functional group are randomly entangled, maybe obtained by thermally decomposing the core of the fibrous structureand carbonizing the sheath of the fibrous structure through heattreatment of the film.

The sheet may be obtained by heat-treating the film at a temperatureranging from a thermal decomposition temperature of the core to acarbonizing temperature of the sheath.

The sheet may be obtained by heat-treating the film at a temperature of600° C. to 900° C. for 1 hr to 3 hr.

According to the present disclosure, a cathode for a lithium air batteryincludes a hollow structure, whereby the transfer of a substance such asoxygen in the cathode is not impeded by lithium peroxide generated andstored upon discharging.

According to the present disclosure, the cathode for a lithium airbattery includes no binder, and thus side reactions due to the binder donot occur.

According to the present disclosure, the cathode for a lithium airbattery has a hollow structure that is relatively short in length, thusincreasing the efficiency of internal space utilization of the cathodeto thereby improve the reversibility of oxidation and reduction in thecathode.

According to the present disclosure, the cathode for a lithium airbattery is configured such that the sheath of the hollow structure canconsist of the nitrogen-doped carbon, thus further promoting theoxidation and reduction in the cathode.

Consequently, the use of the cathode for a lithium air battery accordingto the present disclosure can significantly increase the capacity andlifespan of a lithium air battery.

The effects of the present disclosure are not limited to the foregoing,and should be understood to include all effects that can be reasonablyanticipated from the following description.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 schematically shows a process of manufacturing a cathode for alithium air battery according to the present disclosure.

FIG. 2 schematically shows a portion of a fibrous structure, which maybe obtained during the manufacture of the cathode for a lithium airbattery according to the present disclosure.

FIG. 3A is a field-emission scanning electron microscope (FE-SEM) imageshowing a fibrous polymer prepared in Example 1, FIG. 3B is an FE-SEMimage showing a hollow structure manufactured in Example 1, and FIG. 3Cis an FE-SEM image showing a hollow structure manufactured in Example 2;

FIG. 4 is a graph showing the results of Measurement Example 1.

FIG. 5A is an FE-SEM image showing a hollow structure manufactured inExample 3, and FIG. 5B is an FE-SEM image showing a hollow structurepulverized in Comparative Example 2;

FIG. 6 is a graph showing the results of Measurement Example 2.

FIG. 7A is a graph showing the results of Measurement Example 3 for thelithium air battery of Comparative Example 2, FIG. 7B is a graph showingthe results of Measurement Example 3 for the lithium air battery ofExample 2, and FIG. 7C is a graph showing the results of MeasurementExample 3 for the lithium air battery of Example 3.

FIG. 8A is a graph showing the results of Measurement Example 4 for thelithium air battery of Comparative Example 2, FIG. 8B is a graph showingthe results of Measurement Example 4 for the lithium air battery ofExample 2, and FIG. 8C is a graph showing the results of MeasurementExample 4 for the lithium air battery of Example 3.

DESCRIPTION OF SPECIFIC EMBODIMENTS

The above and other objectives, features and advantages of the presentdisclosure will be more clearly understood from the following preferredembodiments taken in conjunction with the accompanying drawings.However, the present disclosure is not limited to the embodimentsdisclosed herein, and may be modified into different forms. Theseembodiments are provided to thoroughly explain the disclosure and tosufficiently transfer the spirit of the present disclosure to thoseskilled in the art.

Throughout the drawings, the same reference numerals will refer to thesame or like elements. For the sake of clarity of the presentdisclosure, the dimensions of structures are depicted as being largerthan the actual sizes thereof. It will be understood that, althoughterms such as “first”, “second”, etc. may be used herein to describevarious elements, these elements are not to be limited by these terms.These terms are only used to distinguish one element from anotherelement. For instance, a “first” element discussed below could be termeda “second” element without departing from the scope of the presentdisclosure. Similarly, the “second” element could also be termed a“first” element. As used herein, the singular forms are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise.

It will be further understood that the terms “comprise”, “include”,“have”, etc., when used in this specification, specify the presence ofstated features, integers, steps, operations, elements, components, orcombinations thereof, but do not preclude the presence or addition ofone or more other features, integers, steps, operations, elements,components, or combinations thereof. Also, it will be understood thatwhen an element such as a layer, film, area, or sheet is referred to asbeing “on” another element, it can be directly on the other element, orintervening elements may be present therebetween. Similarly, when anelement such as a layer, film, area, or sheet is referred to as being“under” another element, it can be directly under the other element, orintervening elements may be present therebetween.

Unless otherwise specified, all numbers, values, and/or representationsthat express the amounts of components, reaction conditions, polymercompositions, and mixtures used herein are to be taken as approximationsincluding various uncertainties affecting the measurements thatessentially occur in obtaining these values, among others, and thusshould be understood to be modified by the term “about” in all cases.Furthermore, when a numerical range is disclosed in this specification,the range is continuous, and includes all values from the minimum valueof said range to the maximum value thereof, unless otherwise indicated.Moreover, when such a range pertains to integer values, all integersincluding the minimum value to the maximum value are included, unlessotherwise indicated.

FIG. 1 schematically shows a process of manufacturing a cathode for alithium air battery according to the present disclosure.

Specifically, the method of manufacturing a cathode for a lithium airbattery according to the present disclosure includes preparing a fibrouspolymer by electrospinning a spinning solution including a thermallydecomposable polymer (S10), manufacturing a fibrous structure comprisinga core including the fibrous polymer and a sheath including anitrogen-containing polymer by coating the surface of the fibrouspolymer with the nitrogen-containing polymer (S20), subjecting thefibrous polymer to ultrasonication (S30), manufacturing a filmconfigured such that the fibrous structures are randomly entangled(S40), and obtaining a sheet configured such that hollow structuresincluding a carbon material having a nitrogen functional group arerandomly entangled by thermally decomposing the core of the fibrousstructure and carbonizing the sheath thereof through heat treatment ofthe film (S50).

(S10) The fibrous polymer may be obtained by electrospinning a spinningsolution including a thermally decomposable polymer.

The thermally decomposable polymer may be selected from the groupconsisting of polystyrene (PS), poly(methyl methacrylate) (PMMA) andcombinations thereof.

The conditions for the electrospinning process are not particularlylimited, and may be appropriately changed depending on the kind of thethermally decomposable polymer, the specifications of a hollowstructure, which will be described later, etc.

(S20) The fibrous structure may be obtained by coating the surface ofthe fibrous polymer with a nitrogen-containing polymer. FIG. 2 is aperspective view schematically showing a portion of the fibrousstructure. With reference thereto, the fibrous structure 10 includes acore 11 including a fibrous polymer, namely a thermally decomposablepolymer, and a sheath 12 applied on the surface of the core 11 andincluding a nitrogen-containing polymer.

The nitrogen-containing polymer may be selected from the groupconsisting of polydopamine (PDA), polyacrylonitrile (PAN), polypyrrole(PPy), polyaniline (PANI) and combinations thereof.

The fibrous structure may be obtained by adding the fibrous polymer to apolymer solution including any one selected from the group consisting ofa nitrogen-containing monomer, a nitrogen-containing polymer andcombinations thereof and then performing stirring. Specifically, whenthe nitrogen-containing polymer is polydopamine, the fibrous polymer isadded to a polymer solution including polydopamine and/or a dopaminemonomer and stirred, thereby manufacturing a fibrous structure.

The fibrous structure may be configured such that thenitrogen-containing polymer is applied at a thickness of 10 nm to 100 nmon the surface of the fibrous polymer. With reference to FIG. 2, thesheath 12 may be applied at a thickness of 10 nm to 100 nm on the core11.

The process of coating with the nitrogen-containing polymer is notparticularly limited, and may be appropriately performed depending onthe kind of nitrogen-containing polymer, the type of manufacturingdevice, etc. Preferably, the coating with the nitrogen-containingpolymer is repeated several times such that the thickness of thenitrogen-containing polymer applied on the surface of the fibrouspolymer falls in the above numeral range. Specifically, the fibrouspolymer is added to the polymer solution and is sufficiently stirredsuch that a predetermined amount of the nitrogen-containing polymerincluded in the polymer solution is completely applied on the surface ofthe fibrous polymer, after which the nitrogen-containing polymer isre-added in an amount that is the same as or similar to the above amountto the polymer solution, and the surface of the fibrous polymer iscoated with the nitrogen-containing polymer thus re-added. Inparticular, the number of processes of coating with thenitrogen-containing polymer to manufacture a fibrous structure may be 1to 10.

(S30) The present disclosure is not limited thereto, but the fibrousstructure may be subjected to ultrasonication. When the fibrousstructure is excessively long, the length of a hollow structure, whichwill be described later, is also increased, and thus decomposition oflithium peroxide grown in the hollow structure may become inefficient.Hence, when the fibrous structure is cut through ultrasonication, theabove problems may be prevented from occurring.

The specific process for ultrasonication is not particularly limited,and may be appropriately performed under proper conditions depending onthe type of an ultrasonicator, the length of the fibrous structure, etc.

(S40) A film in a predetermined shape and thickness may be manufacturedfrom a plurality of the fibrous structures, which may or may not besubjected to ultrasonication.

The specific process for manufacturing the film is not particularlylimited, and preferably, a solution including the fibrous structure isfiltered such that a film, configured such that the fibrous structuresare randomly entangled, is formed on the filter paper.

As used herein, “randomly entangled” does not mean that the shape,thickness and the like of the film are irregular but means that fibrousstructures constituting the film are disposed in individual directionsor positions without affecting each other, and may be simply interpretedto mean that the fibrous structures are provided in the form of a netstructure.

Moreover, in the present disclosure, no binder is used to form a filmhaving a predetermined shape and thickness including the fibrousstructures. Even when the binder is not used, the fibrous structures areentangled in a net shape, thereby foaming a film having a predeterminedshape and thickness. Accordingly, a final cathode may contain no bindertherein, and side reactions due to the binder may be prevented fromoccurring.

(S50) Finally, the film is heat-treated, whereby the core of the fibrousstructure is thermally decomposed and the sheath thereof is carbonized,ultimately obtaining a sheet configured such that hollow structuresincluding a carbon material having a nitrogen functional group arerandomly entangled. Here, the sheet may be a cathode for a lithium airbattery.

The core of the fibrous structure is formed of the fibrous polymer,which is obtained by electrospinning the thermally decomposable polymer,and is decomposed when heated. Accordingly, the hollow structure may beconfigured such that the center thereof is empty without a corematerial.

The hollow structure may be effectively utilized as a place for storinglithium peroxide generated during the discharge of a lithium airbattery, and may also prevent pore clogging during the operation of alithium air battery, thereby making it easy to decompose lithiumperoxide at the time of recharge by facilitating the transfer of lithiumions and oxygen even in a fully discharged state.

Furthermore, when the temperature for heat treatment is appropriatelyadjusted, the nitrogen-containing polymer constituting the sheath of thefibrous structure is carbonized into a carbon material having a nitrogenfunctional group. Specifically, the film may be heat-treated at atemperature ranging from 600° C. to 900° C. for 1 hr or 3 hr.

Consequently, the sheet is configured such that the hollow structuresincluding the carbon material having the nitrogen functional group arerandomly entangled.

The nitrogen functional group refers to a composite of nitrogen (N),carbon (C) and hydrogen (H) or a composite of nitrogen (N) and carbon(C) present in the carbon material. The nitrogen functional groupincluded in the carbon material may be selected from the groupconsisting of pyridinic nitrogen (pyridinic-N), pyrrolic nitrogen(pyrrolic-N), graphitic nitrogen (graphitic-N) and combinations thereof.Here, pyridinic-N, pyrrolic-N and graphitic-N are configured such thatcarbon (C) is substituted with nitrogen (N) in a carbon array.Specifically, pyridinic-N is configured such that two carbons (C) arelinked to nitrogen (N), pyrrolic-N is configured such that two carbons(C) and one hydrogen (H) are linked to nitrogen (N), and graphitic-N isconfigured such that three carbons (C) are linked to nitrogen (N).

As the nitrogen-containing polymer, polydopamine, represented byChemical Formula 1 below, may be used. When the polydopamine iscarbonized, a carbon material having a nitrogen functional group with apartial structure represented by Chemical Formula 2 below may result.

The graphitic-N is effective at oxygen reduction, which is the mainreaction occurring during discharge of the lithium air battery. Thus,the hollow structure is capable of effectively promoting the oxygenreduction due to nitrogen doping.

The length, thickness and diameter of the hollow structure are notparticularly limited. Preferably, the length thereof is 0.5 μm to 300μm, and more preferably 1 μm to 100 μm. Also, a thickness of 0.05 μm to0.5 μm and a diameter of 2 μm to 10 μm are preferable.

A better understanding of the present disclosure will be given throughthe following examples.

EXAMPLE 1

A spinning solution including 30 wt % of polystyrene was electrospun inethanol for 90 min at a speed of 0.6 mL/hr, a voltage of 18 kV, and aspinning distance of 20 cm, thus obtaining a fibrous polymer. Thesolvent for the spinning solution was dimethyl formaldehyde. FIG. 3A isan FE-SEM image showing the fibrous polymer.

The surface of the fibrous polymer was coated with a nitrogen-containingpolymer, thus manufacturing a fibrous structure. The fibrous polymer wasplaced in a dopamine solution comprising 450 mg of a dopamine monomer,300 mL of distilled water, and 1.8 g of tris and stirring was thenperformed for about 2 hr. This procedure was carried out once.

The solution containing a plurality of the fibrous structures dispersedtherein were slowly poured into a vacuum filter device, followed by slowfiltration, whereby a film configured such that a plurality of thefibrous structures were randomly entangled was formed on the filterpaper. The film was dried at about 80° C. for about 12 hr.

The film was heat-treated in an inert gas atmosphere at 900° C. forabout 1 hr, whereby the fibrous polymer (polystyrene) was thermallydecomposed and simultaneously the nitrogen-containing polymer(polydopamine) was carbonized. Finally, a cathode in a sheet formcomprising a plurality of the hollow structures was obtained. FIG. 3B isan FE-SEM image showing the hollow structure of Example 1.

A CR2032 coin cell was manufactured using the above cathode, a lithiummetal anode having a thickness of 200 μm, a separator made of glassfiber, and an electrolyte solution including tetraethylene glycoldimethylether (TEGDME) added with 1.0 M bis(trifluoromethane)sulfonimidelithium salt (LiTFSI) as a lithium salt.

EXAMPLE 2

Compared to Example 1, a fibrous structure was manufactured byperforming coating of the surface of a fibrous polymer with anitrogen-containing polymer about five times. Specifically, the fibrouspolymer was placed in a dopamine solution comprising 450 mg of adopamine monomer, 300 mL of distilled water, and 1.8 g oftris(hydroxymethyl)aminomethane (tris), and shaking was then performedfor about 2 hr. Thereafter, the above dopamine solution was re-added,and the above procedure was repeated. The process of coating with thenitrogen-containing polymer was carried out a total of five times.

In addition thereto, a coin cell (a lithium air battery) wasmanufactured using the same material and process as in Example 1. FIG.3C is an FE-SEM image showing the hollow structure of Example 2.

COMPARATIVE EXAMPLE 1

A polyacrylonitrile-carbonized electrode, which is a typical binder-freefibrous structure electrode, was used as a control.

A 10 wt % polyacrylonitrile/dimethylformaldehyde solution waselectrospun using a drum collector for 90 min at a speed of 0.6 mL/hr, avoltage of 18 kV, and a spinning distance of 20 cm, and was thencarbonized through heat treatment for 1 hr in air at 230° C. and for 1hr in an inert gas atmosphere at 900° C.

After completion of the heat treatment, the carbonized polyacrylonitrilesheet was punched to a certain size and was used for a cathode, and alithium air battery was manufactured in the same manner as in Example 1.

MEASUREMENT EXAMPLE 1

The lithium air battery of each of Examples 1 and 2 and ComparativeExample 1 was subjected to initial charge/discharge testing. When aconstant current of 50 mA/g_(carbon) was applied to the lithium airbattery, a discharge capacity reaching 2 V, which is a capacity cutoffcondition, and a charge capacity reaching 4.5 V upon recharging weremeasured.

FIG. 4 is a graph showing the behavior curve of theconstant-current-discharged/charged lithium air battery. With referenceto FIG. 4, the discharge capacity was measured to be 2,291mAh/g_(carbon) in Comparative Example 1, 4,957 mAh/g_(carbon) in Example1 and 5,902 mAh/g_(carbon) in Example 2. Also, the reversibility was3.9% in Comparative Example 1 but was improved to 59.7% in Example 1 and71.0% in Example 2.

According to the present disclosure, the cathode comprising the hollowstructure exhibited increased discharge capacity and high reactionreversibility compared to the cathode comprising no hollow structure,from which the cathode comprising the hollow structure is evaluated tobe more favorable for the operation of a lithium air battery.

EXAMPLE 3

A fibrous structure was manufactured using the same material and processas in Example 1. Thereafter, the fibrous structure was cut in lengththrough ultrasonication. Specifically, the fibrous structure wasdispersed in 300 mL of distilled water, and the length thereof was cutthrough ultrasonication for about 2 hr.

In addition thereto, a coin cell (a lithium air battery) wasmanufactured in the same manner as in Example 1. FIG. 5A is an FE-SEMimage showing the cut hollow structure of Example 3.

COMPARATIVE EXAMPLE 2

In order to evaluate the structural effect resulting from cutting thelength of the fibrous structure through ultrasonication and the effectof not using a binder, the cathode comprising the hollow structureobtained in Example 1 was mechanically pulverized, and a cathode in asheet form was manufactured again using a binder.

Specifically, a predetermined amount of a polyvinylidene fluoride binderwas added to an N-methylpyrrolidone solvent and dispersed to give abinder solution. The pulverized hollow structure and the binder solutionwere mixed such that the weight ratio of the pulverized hollow structureand the binder was 2.5:1, and stirring was sufficiently performed.

The stirred product was uniformly applied to a predetermined thicknesson a fixed carbon paper substrate using a vacuum-spraying device.Electrode loading was adjusted at about 1.5 mg of carbon per cm².

The electrode thus manufactured was dried at 100° C. for 12 hr in avacuum, thereby completely removing the solvent. The completely driedelectrode was blanked to a certain size, and a lithium air battery wasmanufactured in the same manner as in Example 1.

FIG. 5B is an FE-SEM image showing the mechanically pulverized hollowstructure of Comparative Example 2.

MEASUREMENT EXAMPLE 2

The lithium air battery of each of Examples 2 and 3 and ComparativeExample 2 was subjected to initial charge/discharge testing. When aconstant current of 50 mA/g_(carbon) was applied to the lithium airbattery, a discharge capacity reaching 2 V, which is a capacity cutoffcondition, and a charge capacity reaching 4.5 V upon recharging weremeasured.

FIG. 6 is a graph showing the behavior curve of theconstant-current-discharged/charged lithium air battery. With referenceto FIG. 6, the discharge capacity was measured to be 2,537mAh/g_(carbon) in Comparative Example 2, 5,904 mAh/g_(carbon) in Example2 and 4,900 mAh/g_(carbon) in Example 3. Also, the reversibility was66.3% in Comparative Example 2 but was improved to 71.0% in Example 2and 95.4% in Example 3.

According to the present disclosure, the use of the hollow structure,without the binder, increased the capacity and reduced the overvoltageeffect. Furthermore, the hollow structure was cut to have a much shorterlength, whereby the specific surface area of the electrode was reduceddue to a decrease in the space between fibers, and thus the totalcapacity was decreased by about 17%, but it became easy to access thehollow structure, ultimately increasing the efficiency of internal spaceutilization thereof, thereby overcoming the existing limitedreversibility problem.

MEASUREMENT EXAMPLE 3

The lithium air battery of each of Examples 2 and 3 and ComparativeExample 2 was measured for charge/discharge characteristics at differentrates. When different constant currents of 50, 100, and 300mA/g_(carbon) were applied to the lithium air battery, a dischargecapacity reaching 2 V, which is a capacity cutoff condition, and acharge capacity reaching 4.5 V upon recharging were measured.

With reference to FIG. 7A, as the applied current density was increasedto 100 and 300 mA/g_(carbon), in Comparative Example 2, the dischargecapacity was decreased by 33.3% and 75.5% and the reversibility was alsolow, to levels of 52.5% and 31.5%.

In contrast, with reference to FIG. 7B, in Example 2, the dischargecapacity was decreased by about 37.6% at 100 mA/g_(carbon) and by 60.9%at 300 mA/g_(carbon), and the reversibility was also low, to levels of50.9% and 56.6% at respective current densities.

With reference to FIG. 7C, in Example 3, the extent of decrease indischarge capacity with an increase in current density was significantlyreduced, and thus the discharge capacity was decreased by only about9.2% at a current density of 100 mA/g_(carbon) and by about 30% at ahigh current density of 300 mA/g_(carbon). The reversibility was greatlyimproved to 94.7% and 92.2% at respective current densities.

When the length of the hollow structure is cut as described in Example3, it is easier to transfer the reactants into and out of the hollowstructure, whereby the lithium air battery can be found to operateuniformly using the electrode structure even under fastdischarging/charging conditions.

MEASUREMENT EXAMPLE 4

The lifespan of the lithium air battery of each of Examples 2 and 3 andComparative Example 2 was measured. Under the condition that thecapacity of the lithium air battery was limited to 500 mAh/g_(carbon)and a constant current of 100 mA/g_(carbon) was applied, discharging andcharging were repeated and the lifespan of the battery was measured.With reference to FIG. 8A, in Comparative Example 2, the lifespan wasmaintained up to 11 cycles. As shown in FIG. 8B, in Example 2, thelifespan was maintained up to 45 cycles, and as shown in FIG. 8C, inExample 3, the battery operated stably up to 68 cycles.

According to the present disclosure, as is apparent from MeasurementExample 2, when the cut hollow structure was used, compared to whenusing the uncut hollow structure, the discharge capacity was decreasedbut the transfer of the reactants through the hollow structure becameeasier and the efficiency of internal space utilization thereof wasincreased, thus facilitating the generation and decomposition of lithiumperoxide, resulting in a lithium air battery having a prolongedlifespan.

Although specific embodiments of the present disclosure have beendescribed with reference to the accompanying drawings, those skilled inthe art will appreciate that the present disclosure may be embodied inother specific forms without changing the technical spirit or essentialfeatures thereof. Thus, the embodiments described above should beunderstood to be non-limiting and illustrative in every way.

What is claimed is:
 1. A method of manufacturing a cathode for a lithiumair battery, comprising: preparing a fibrous polymer by electrospinninga spinning solution including a thermally decomposable polymer;manufacturing a fibrous structure comprising a core including thefibrous polymer and a sheath including a nitrogen-containing polymer bycoating a surface of the fibrous polymer with the nitrogen-containingpolymer; manufacturing a film comprising a plurality of the fibrousstructures, wherein the fibrous structures are randomly entangled; andobtaining a sheet by heat-treating the film.
 2. The method of claim 1,wherein the thermally decomposable polymer comprises one selected fromthe group consisting of polystyrene (PS), poly(methyl methacrylate)(PMMA) and combinations thereof.
 3. The method of claim 1, wherein thenitrogen-containing polymer comprises at least one selected from thegroup consisting of polydopamine (PDA), polyacrylonitrile (PAN),polypyrrole (PPy), polyaniline (PANI) and combinations thereof.
 4. Themethod of claim 1, wherein the manufacturing the fibrous structurefurther comprising: adding the fibrous polymer to a polymer solutionincluding one selected from the group consisting of anitrogen-containing monomer, the nitrogen-containing polymer andcombinations thereof; and performing stirring.
 5. The method of claim 1,wherein a thickness of the nitrogen-containing polymer on the surface ofthe fibrous polymer is from about 10 nm to about 100 nm.
 6. The methodof claim 5, wherein the coating of the surface of the fibrous polymerwith the nitrogen-containing polymer is performed from about 1 to about10 times.
 7. The method of claim 1, further comprising subjecting thefibrous structure to ultrasonication before manufacturing the film. 8.The method of claim 1, wherein the sheet is obtained by thermallydecomposing the core of the fibrous structure and carbonizing the sheathof the fibrous structure through the heat-treating of the film to yielda sheet comprising a plurality of hollow structures that are randomlyentangled and each of the hollow structures including a carbon materialhaving a nitrogen functional group.
 9. The method of claim 1, whereinthe sheet is obtained by heat-treating the film at a temperature rangingfrom a thermal decomposition temperature of the core to a carbonizingtemperature of the sheath.
 10. The method of claim 1, wherein theheat-treating is conducted at a temperature of about 600° C. to about900° C. for about 1 hr to about 3 hr.
 11. The method of claim 8, whereinthe nitrogen functional group includes graphic nitrogen (graphitic-N).12. The method of claim 8, wherein a length of one or more of the hollowstructures is about 0.5 μm to about 300 μm.
 13. The method of claim 8,wherein a diameter of one or more of the hollow structures is about 2 μmto about 10 μm.